Crossing ISI cancellation

ABSTRACT

An apparatus comprising an inter symbol interference (ISI) cancellation circuit and a detector circuit. The inter symbol interference (ISI) cancellation circuit may be configured to minimize ISI at data sampling and crossing sampling points in a symbol interval of an input signal. The detector circuit may be configured to generate data samples and crossing samples at the data sampling and crossing sampling points in the symbol interval of the input signal.

This application relates to U.S. Ser. No. 13/073,077, filed Mar. 28,2011, which is incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to communications generally and, moreparticularly, to a method and/or apparatus for implementing crossing ISIcancellation.

BACKGROUND OF THE INVENTION

Clock and Data Recovery (CDR) circuits form a part ofSerializer/Deserializer (SerDes) receivers. The CDR circuits track thephase of a sampling clock based on some criterion, such as minimizing aMean-Squared-Error (MSE). Conventional CDR circuits are commonlydesigned to achieve low target bit-error-ratios (BER) on the order of10⁻¹² to 10⁻¹⁵ errors per bit. One category of CDR circuits commonlyused is bang-bang CDR. Bang-bang CDR is widely used in SerDes circuitsdue to superior jitter tolerance and insensitivity to the Nyquist datapattern (i.e., 1010 . . . ).

Due to impairments in communication channels, previous and futuresymbols can have interference with a current symbol. Such interferenceis called inter symbol interference (ISI). From data samples obtained bysampling at the middle of a data eye, it is well understood that the ISIto the data samples needs to be reduced by means of linear or nonlinearequalization. For example, decision feedback equalization (DFE) can beused to remove the ISI at the data sample from the previous symbols.However, inter symbol interference at zero crossings was not wellunderstood in the past. Consequently, a systematic approach to addressISI at zero crossings does not exist.

Because of the lack of a systematic approach, the ISI at the zerocrossings is often not addressed properly. While the DFE feedbackwaveform has the intended magnitude at the data sample, the feedbackwaveform has only a portion of that magnitude at the zero crossingbefore or after the data sampling point. The feedback at the datasampling point is often related to the ISI the DFE is designed toremove. However, the feedback at the zero crossing is unrelated to theISI at that zero crossing, causing the ISI there to be worse or betterin an uncontrolled manner. The amount of feedback at the zero crossingdepends on the implementation and is often not easily controlled.

Clock and data recovery (CDR) using the zero crossing sample (such asBang-bang CDR) can be affected by the residual zero-crossing ISI. Thezero crossings of the equalized eye can have a wider region due to theresidual zero-crossing ISI (quantified as the horizontal eye margin).When jitter is present, the data sampling can occur near the zerocrossing region, leading to bit errors.

In general, maximizing both the vertical eye margin and the horizontalmargin can be difficult. An equalization method that minimizes the ISIat the data samples often maximizes the vertical eye margin, while anequalization method that minimizes the ISI at the zero crossing samplesleads to the best horizontal eye margin. A trade off has to be made as aresult.

It would be desirable to have a method of ISI cancellation that balancesminimization of ISI at the data samples with minimization of ISI at thezero crossings.

SUMMARY OF THE INVENTION

The present invention concerns an apparatus comprising an inter symbolinterference (ISI) cancellation circuit and a detector circuit. Theinter symbol interference (ISI) cancellation circuit may be configuredto minimize ISI at data sampling and crossing sampling points in asymbol interval of an input signal. The detector circuit may beconfigured to generate data samples and crossing samples at the datasampling and crossing sampling points in the symbol interval of theinput signal.

The objects, features and advantages of the present invention includeproviding a method and/or apparatus for implementing crossing ISIcancellation that may (i) minimize data ISI and crossing ISI at the sametime, (ii) maximize both vertical and horizontal margins, (iii) enablegood performance, and/or (iv) provide robustness.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the presentinvention will be apparent from the following detailed description andthe appended claims and drawings in which:

FIG. 1 is a diagram illustrating an example embodiment in accordancewith the present invention;

FIG. 2 is a diagram illustrating another example embodiment inaccordance with the present invention;

FIG. 3 is a diagram illustrating still another example embodiment inaccordance with the present invention;

FIG. 4 is a diagram illustrating a combined data and crossing ISIcanceler in accordance with yet another example embodiment of thepresent invention;

FIG. 5 is a timing diagram illustrating example feedback waveforms ofFIG. 4;

FIG. 6 is a diagram illustrating a simplified implementation of thecombined data and crossing ISI canceler of FIG. 4;

FIG. 7 is a diagram illustrating a 2T version of the combined data andcrossing ISI canceler of FIG. 6;

FIG. 8 is a diagram illustrating an example data ISI canceler inaccordance with an embodiment of the present invention;

FIG. 9 is a diagram illustrating another example data ISI canceler inaccordance with an embodiment of the present invention;

FIG. 10 is a diagram illustrating an example crossing ISI cancelerimplemented in accordance with an embodiment of the present invention;

FIG. 11 is a diagram illustrating an example ISI canceling crossingdetector implemented in accordance with another embodiment of thepresent invention;

FIG. 12 is a diagram illustrating example crossing ISI canceler anddetector circuits implemented in accordance with yet another exampleembodiment of the present invention;

FIG. 13 is a diagram illustrating a crossing ISI canceler in accordancewith still another example embodiment of the present invention;

FIG. 14 is a diagram illustrating a crossing ISI canceler in accordancewith another example embodiment of the present invention;

FIG. 15 is a flow diagram illustrating a process in accordance with anembodiment of the present invention;

FIG. 16 is a flow diagram illustrating a process in accordance withanother embodiment of the present invention; and

FIG. 17 is a diagram illustrating a context in which embodiments of thepresent invention may be implemented.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a block diagram of a circuit 100 is shownillustrating a data inter symbol interference (ISI) canceler andcrossing inter symbol interference (ISI) canceler arranged in accordancewith an example embodiment of the present invention. In one example, ISIcancellation at a data input eye and zero crossings of an input signalmay be performed simultaneously (in parallel). For example, the circuit100 may comprise a block (or circuit) 102, a block (or circuit) 104, ablock (or circuit) 106, and a block (or circuit) 108. The circuits102-108 may represent modules and/or blocks that may be implemented ashardware, software, a combination of hardware and software, or otherimplementations. The block 102 may implement a data ISI canceler. Theblock 104 may implement a crossing ISI canceler. The blocks 106 and 108may implement detectors (e.g., symbol detector, crossing detector, bitslicer, etc.).

An input signal (e.g., a signal received via a communication channel)may be presented to an input of the circuit 102 and an input of thecircuit 104. Depending upon the design criteria of a particularimplementation, the input signal may comprise an analog signal (e.g.,R(t)) or a digital signal (e.g., RK). In one example, some form ofprocessing (e.g., linear equalization, etc.) may already have beenperformed on the input signal. The circuit 102 may have an output thatmay present a first intermediate signal to an input of the circuit 106.The circuit 104 may have an output that may present a secondintermediate signal to an input of the circuit 108. The circuit 106 maybe configured to generate a data sample signal (e.g., DK) in response tothe first intermediate signal. The circuit 108 may be configured togenerate a crossing sample signal (e.g., XK) in response to the secondintermediate signal. The data and crossing sample signals DK and XK maybe used by a following circuit to recover data from the input signal.For example, a clock and data recovery circuit (CDR), such as abang-bang CDR circuit may be configured to utilize the data and crossingsample signals DK and XK generated by the circuit 100. The arrangementof the circuit 102 and 104 in parallel generally avoids the circuit 102and the circuit 104 interfering with each other.

Referring to FIG. 2, a block diagram of a circuit 110 is shownillustrating a data inter symbol interference (ISI) canceler andcrossing inter symbol interference (ISI) canceler arranged in accordancewith another example embodiment of the present invention. In oneexample, the crossing ISI canceler and the data ISI canceler may beconnected in series. For example, the crossing ISI canceler may bepositioned at an output of the data ISI canceler, so that what is insidethe data ISI canceler may impact the crossing ISI canceler, but not viceversa. For example, a portion of a decision feedback equalization (DFE)feedback may be present at a crossing sampling point.

The circuit 110 may comprise a block (or circuit) 112, a block (orcircuit) 114, a block (or circuit) 116, and a block (or circuit) 118.The circuits 112-118 may represent modules and/or blocks that may beimplemented as hardware, software, a combination of hardware andsoftware, or other implementations. The block 112 may implement a dataISI canceler. The block 114 may implement a crossing ISI canceler. Theblocks 116 and 118 may implement detectors (e.g., a symbol detector,crossing detector, bit slicer, etc.).

An input signal (e.g., a signal received via a communication channel)may be presented to an input of the circuit 112. Depending upon thedesign criteria of a particular implementation, the input signal maycomprise an analog signal (e.g., R(t)) or a digital signal (e.g., RK).In one example, some form of processing (e.g., linear equalization,etc.) may already have been performed on the input signal. The circuit112 may have an output that may present a first intermediate signal toan input of the circuit 114 and an input of the circuit 116. The circuit114 may have an output that may present a second intermediate signal toan input of the circuit 118. The circuit 116 may be configured togenerate a data sample signal (e.g., DK) in response to the firstintermediate signal. The circuit 118 may be configured to generate acrossing sample signal (e.g., XK) in response to the second intermediatesignal. The data and crossing sample signals DK and XK may be used by asubsequent circuit to recover data from the input signal. For example, aclock and data recovery circuit (CDR), such as a bang-bang CDR circuitmay be configured to utilize the data and crossing sample signals DK andXK generated by the circuit 110.

Referring to FIG. 3, a block diagram of a circuit 120 is shownillustrating a data inter symbol interference (ISI) canceler andcrossing inter symbol interference (ISI) canceler arranged in accordancewith yet another example embodiment of the present invention. In oneexample, the crossing ISI canceler and the data ISI canceler may beconnected in series. For example, the data ISI canceler may bepositioned at an output of the crossing ISI canceler, so that what isinside the crossing ISI canceler may impact the data ISI canceler, butnot vice versa. For example, a portion of a crossing cancellationfeedback waveform may be present at a data sampling point.

In one example, the circuit 120 may comprise a block (or circuit) 122, ablock (or circuit) 124, a block (or circuit) 126, and a block (orcircuit) 128. The circuits 122-128 may represent modules and/or blocksthat may be implemented as hardware, software, a combination of hardwareand software, or other implementations. The block 122 may implement acrossing ISI canceler. The block 124 may implement a data ISI canceler.The blocks 126 and 128 may implement detectors (e.g., a symbol detector,crossing detector, bit slicer, etc.).

An input signal (e.g., a signal received via a communication channel)may be presented to an input of the circuit 122. Depending upon thedesign criteria of a particular implementation, the input signal maycomprise an analog signal (e.g., R(t)) or a digital signal (e.g., RK).In one example, some form of processing (e.g., linear equalization,etc.) may already have been performed on the input signal. The circuit122 may have an output that may present a first intermediate signal toan input of the circuit 124 and an input of the circuit 126. The circuit124 may have an output that may present a second intermediate signal toan input of the circuit 128. The circuit 126 may be configured togenerate a crossing sample signal (e.g., XK) in response to the firstintermediate signal. The circuit 128 may be configured to generate adata sample signal (e.g., DK) in response to the second intermediatesignal. The data and crossing sample signals DK and XK may be used by asubsequent circuit to recover data from the input signal. For example, aclock and data recovery circuit (CDR), such as a bang-bang CDR circuitmay be configured to utilize the data and crossing sample signals DK andXK generated by the circuit 120.

Referring to FIG. 4, a block diagram of a circuit 130 is shownillustrating an inter symbol interference (ISI) canceler and a data andcrossing detector arranged in accordance with yet another exampleembodiment of the present invention. In one example, data ISIcancellation and crossing ISI cancellation may be performed by sharedcircuitry. Data and crossing detectors may be clocked with respectiveclock signals (e.g., CLK0, CLK90, etc.).

In one example, the circuit 130 may comprise a block (or circuit) 131, ablock (or circuit) 132, and a number of blocks (or circuits) 133 a-133n. The block 131 may comprise a block (or circuit) 134 and a number ofblocks (or circuits) 135 a-135 n. The block 132 may comprise a block (orcircuit 136 and a block (or circuit) 137. The circuits 131-137 mayrepresent modules and/or blocks that may be implemented as hardware,software, a combination of hardware and software, or otherimplementations. The circuit 131 may implement an ISI canceler. Thecircuit 132 may implement a data and crossing detector. The circuits 133a-133 n may be implemented, in one example, as storage or delay elements(e.g., shift registers, sample and hold elements, etc.). The circuit 134may be implemented as an adder. The circuits 135 a-135 n may beimplemented, in one example, as multipliers. The circuit 136 mayimplement a crossing detector. The circuit 137 may implement a datasample detector (e.g., a symbol detector, bit slicer, etc.).

An input signal (e.g., a signal received via a communication channel)may be presented to an input of the circuit 131. Depending upon thedesign criteria of a particular implementation, the input signal maycomprise an analog signal (e.g., R(t)) or a digital signal (e.g., RK).In one example, some form of processing (e.g., linear equalization,etc.) may already have been performed on the input signal. The circuit131 may have an output that may present an intermediate signal to aninput of the circuit 132 and a number of inputs that may receive anumber of feedback signals. The number of feedback signals may comprisepreceding data samples (e.g., DK(1), . . . DK(N)). The circuit 131 maybe configured to generate the intermediate signal in response to theinput signal, the number of feedback signals DK(1), . . . , DK(N), and anumber of tap weight signals (e.g., W1, . . . , WN).

The circuit 132 may have a first output that may present a crossingsample signal (e.g., XK) and a second output that may present a datasample signal (e.g., DK). The circuit 132 may be configured to generatethe crossing sample signal XK and the data sample signal DK in responseto the intermediate signal and one or more clock signals. For example,the crossing sample signal XK may be generated in response to theintermediate signal and a first clock signal (e.g., CLK90). The datasample signal DK may be generated in response to the intermediate signaland a second clock signal (e.g., CLK0). The clock signals (e.g., CLK0,CLK90, etc.) may differ from one another with respect to phase. The dataand crossing sample signals DK and XK may be used by a subsequentcircuit to recover data from the input signal. For example, a clock anddata recovery circuit (CDR), such as a bang-bang CDR circuit may beconfigured to utilize the data and crossing sample signals DK and XKgenerated by the circuit 130.

The data sample signal DK may be presented to an input of the circuit133 a. An output of the circuit 133 a may present the data sample DK(1)to an input of the circuit 133 b. The circuit 133 b may have an outputthat may present the data sample DK(2). The remaining circuit 133 c-133n may be configured similarly to the circuit 133 a and 133 b to generatethe data sample signals DK(3, . . . , DK(N).

The circuit 134 may be configured to generate the intermediate signal inresponse to the input signal and a feedback signal. The feedback signalmay be generated in response to the data sample signals DK(1), . . . ,DK(N) and the tap weight signals W1 through WN. For example, each of thecircuits 135 a-135 n may be configured to weight a respective one of thedata sample signals DK(1), . . . , DK(N) based upon a respective one ofthe tap weight signals W1, . . . , WN. For example, the circuits 135a-135 n may multiply the respective data sample signals DK(1), . . . ,DK(N) by the respective tap weight signals W1, . . . , WN to generate arespective component of the feedback signal used by the circuit 134 togenerate the intermediate signal. In one example, the tap weight signalsW1, . . . , WN may be implemented in accordance with the exampleillustrated in FIG. 5. In one example, N summing nodes may beimplemented, and the circuit 136 and 137 may run at nT to facilitateimplementation with lower speed detector circuits. The circuits 136 and137 may also be combined into one detector, which runs at double therate of the circuits 136 and 137 to generate the signals XK and DK,alternatively (described below in connection with FIG. 6).

Referring to FIG. 5, timing diagrams 138 and 139 are shown illustratingexample feedback waveforms of a tap (e.g., W1) of FIG. 4. The waveform138 generally corresponds to an embodiment of a double rate canceler.The tap feedback signal W1 may behave similarly to a DFE feedbackwaveform, except that the waveform W1 may settle in T/2, rather than oneT, where T represents a symbol interval. The waveform W1 may alsodischarge and settle to H1 within the next T/2. For example, W1 maysettle to H0.5 at the crossing sample and settle to H1 at the datasample, allowing a single canceler to function as the crossing ISIcanceler at the crossing sample and the data ISI canceler at the datasample. The waveform 139 generally illustrates an embodiment where thefeedback waveform may be carefully controlled to have two-stagesettling. Although the tap feedback signal W1 may not be set or reset asfast as that in a double rate canceler, here the tap feedback signal W1may settle to H0.5 at the crossing sample and H1 at the data sample,achieving the same effect.

Referring to FIG. 6, a diagram of a circuit 140 is shown illustratinganother example of data ISI cancellation and crossing ISI cancellationbeing performed by shared circuitry. In one example, the circuit 140 maycomprise a block (or circuit) 141, a block (or circuit) 143, and anumber of blocks (or circuits) 145 a-145 n. The circuit 141 may comprisea block (or circuit) 147 and a number of blocks (or circuits) 149 a-149n. The circuits 141-149 may represent modules and/or blocks that may beimplemented as hardware, software, a combination of hardware andsoftware, or other implementations. The circuit 141 may implement an ISIcanceler. The circuit 143 may implement a detector (e.g., a symboldetector, bit slicer, etc.). The circuits 145 a-145 n may beimplemented, in one example, as storage or delay elements (e.g., shiftregisters, sample and hold elements, etc.). The circuit 147 may beimplemented as an adder. The circuits 149 a-149 n may be implemented, inone example, as multipliers.

An input signal (e.g., a signal received via a communication channel)may be presented to an input of the circuit 141. Depending upon thedesign criteria of a particular implementation, the input signal maycomprise an analog signal (e.g., R(t)) or a digital signal (e.g., RK).In one example, some form of processing (e.g., linear equalization,etc.) may already have been performed on the input signal. The circuit141 may have an output that may present an intermediate signal to aninput of the circuit 143, and a number of inputs that may receive anumber of feedback signals. The number of feedback signals may comprisepreceding data samples (e.g., DK(1), . . . , DK(N)). The circuit 141 maybe configured to generate the intermediate signal in response to theinput signal, the number of feedback signals DK(1), . . . , DK(N), and anumber of tap weight signals (e.g., H1, . . . , HN). The signalpresented to the input of the circuit 143 may be generated by a samplerconfigured to sample the intermediate signal in response to a clocksignal (e.g., CLK). The clock signal CLK may have a frequency that istwice the frequency of the clock signal CLK0.

The circuit 143 may have an output that may present a signal (e.g., YK).The signal YK may be presented to an input of the circuit 145 a. Anoutput of the circuit 145 a may present the sample YK(1) to an input ofthe circuit 145 b. The circuit 145 b may have an output that may presentthe data sample YK(2). The remaining circuit 145 c-145 n may beconfigured similarly to the circuit 145 a and 145 b to generate the datasample signals YK(3, . . . , YK(N).

The circuit 147 may be configured to generate the intermediate signal inresponse to the input signal and a feedback signal. The feedback signalmay be generated in response to the signals YK(1, . . . , YK(N) and thetap weight signals H1 through HN. For example, each of the circuits 149a-149 n may be configured to weight a respective one of the data samplesignals YK(1), . . . , YK(N) based upon a respective one of the tapweight signals H1, . . . , HN. For example, the circuits 149 a-149 n maymultiply the respective data sample signals YK(1), . . . , YK(N) by therespective tap weight signals H1, . . . , HN to generate a respectivecomponent of the feedback signal used by the circuit 147 to generate theintermediate signal.

In one example, a single detector may be used, running at a double rate,to obtain both the crossing sample XK and the data sample DK. The outputof the single detector (e.g., YK) generally comprises two samples (e.g.,XK and DK) in one symbol interval. In general, the storage or delayelements 145 a-145 n are still shifted by one T, rather than T/2. Thecircuit 140 generally corresponds to a special case of the circuit 130,described above in connection with FIG. 4. However, DFE tap weights H1through HN are used instead of the feedback signals W1 through WN. Inaddition, the crossing sample signal XK and delayed versions of thecrossing sample signal XK are fed back to cancel the crossing ISI. Thecircuit 130 may utilize the expression −H0.5*DK(1)−H1.5*DK(2)− . . .−HN−0.5*DK(N) to cancel the crossing ISI, while the circuit 140generally implements the equation −H1*XK(1)−H2*XK(2)− . . . −HN*XK(N) tocancel the crossing ISI. The method illustrated in FIG. 4 is generallycorrect because it is based upon the formula of crossing ISI. However,the method illustrated in FIG. 6 is generally simpler to implement.

Referring to FIG. 7, a diagram of a circuit 150 is shown illustratinganother example of data ISI cancellation and crossing ISI cancellationbeing performed by shared circuitry. The circuit 150 may comprise ablock (or circuit) 151, a block (or circuit) 152, a block (or circuit)153, a block (or circuit) 154, a number of blocks (or circuits) 155a-155 n, a number of blocks (or circuits) 156 a-156 n, a number ofblocks (or circuits) 157 a-157 n, and a number of blocks (or circuits)158 a-158 n. The circuits 151-138 may represent modules and/or blocksthat may be implemented as hardware, software, a combination of hardwareand software, or other implementations. The circuits 151 and 152 may beimplemented, in one example, as adders. The circuit 153 may implement adata detector (e.g., a symbol detector, bit slicer, etc.). The circuit154 may implement a crossing detector. The circuits 155 a-155 n and 156a-156 n may be implemented, in one example, as storage or delay elements(e.g., shift registers, sample and hold elements, etc.). The circuits157 a-157 n and 158 a-158 n may be implemented, in one example, asmultipliers.

An input signal (e.g., a signal received via a communication channel)may be presented to an input of the circuit 151 and an input of thecircuit 152. Depending upon the design criteria of a particularimplementation, the input signal may comprise an analog signal (e.g.,R(t)) or a digital signal (e.g., RK). In one example, some form ofprocessing (e.g., linear equalization, etc.) may already have beenperformed on the input signal. The circuit 151 may have an output thatmay present a first intermediate signal to an input of the circuit 153,and an input that may receive a first feedback signal. The circuit 151may be configured to generate the first intermediate signal in responseto the input signal and the first feedback signal. The circuit 152 mayhave an output that may present a second intermediate signal to an inputof the circuit 154, and an input that may receive a second feedbacksignal. The circuit 152 may be configured to generate the secondintermediate signal in response to the input signal and the secondfeedback signal. The circuit 153 may have an output that may present adata sample signal (e.g., DK). The circuit 154 may have an output thatmay present a crossing sample signal (e.g., XK).

The signal DK may be presented to an input of the circuit 155 a. Anoutput of the circuit 155 a may present a signal (e.g., DK(1)) to aninput of the circuit 155 b and an input of the circuit 157 a. Thecircuit 157 a may have a second input that may receive a tap weight(e.g., H1). The circuits 155 b-155 n and 157 b-157 n may be configuredsimilarly to the circuits 155 a and 157 a and the respective signalsDK(2)-DK(N) and tap weights H2-HN utilized accordingly. The circuits 157a-157 n may have respective outputs that may present signals that may becombined to form the first feedback signal.

The signal XK may be presented to an input of the circuit 156 a. Anoutput of the circuit 156 a may present a signal (e.g., XK(1)) to aninput of the circuit 156 b and an input of the circuit 158 a. Thecircuit 158 a may have a second input that may receive the tap weight(e.g., H1). The circuits 156 b-156 n and 158 b-158 n may be configuredsimilarly to the circuits 156 a and 158 a and the respective crossingsample signals XK(2)-XK(N) and tap weights H2 -HN utilized accordingly.The circuits 158 a-158 n may have respective outputs that may presentsignals that may be combined to form the second feedback signal.

The circuit 153 may be configured to generate the data sample signal DKin response to the first intermediate signal and a first clock signal(e.g., CLK0). The circuit 154 may be configured to generate the crossingsample signal XK in response to the second intermediate signal and asecond clock signal (e.g., CLK90). The data and crossing sample signalsDK and XK may be used by a subsequent circuit to recover data from theinput signal. For example, a clock and data recovery circuit (CDR), suchas a bang-bang CDR circuit may be configured to utilize the data andcrossing sample signals DK and XK generated by the circuit 150.

In one example, the shared circuit may be implemented using the 2Tarchitecture illustrated. However, an nT architecture may be implementedsimilarly to meet the design criteria of a particular implementation.Further parallelization may be used to allow the two detectors 153 and154 to run at a lower rate. The odd path (e.g., detector 153) generallypresents DK as an output, while the even path (e.g., detector 154)generally presents XK as an output. In general, the two paths do notinterconnect as in a typical 2T-DFE.

The examples illustrated in connection with FIGS. 4-7 generally usedirect feedback only. However, as would be apparent to a person ofordinary skill in the art in light of the teachings contained herein,the circuits illustrated in FIGS. 4, 6, and 7 may also be implementedfully or partially unrolled (as illustrated below in connection withFIGS. 11-13).

Referring to FIG. 8, a diagram of a circuit 160 is shown illustrating anexample of a data ISI canceler in accordance with an embodiment of thepresent invention. The circuit 160 is generally configured as a fullrate decision feedback equalizer (DFE). In one example, the circuit 160may comprise a block (or circuit) 161, a block (or circuit) 163, and anumber of blocks (or circuits) 165 a-165 n. The block 161 may comprise ablock (or circuit) 167 and a number of blocks (or circuits) 169 a-169 n.The circuits 161-169 n may represent modules and/or blocks that may beimplemented as hardware, software, a combination of hardware andsoftware, or other implementations. The circuit 161 may implement a dataISI canceler. The circuit 163 may implement a detector. The circuit 165a-165 n may be implemented as storage or delay elements (e.g., shiftregisters, sample and hold elements, etc.). The circuit 167 may beimplemented as an adder. The circuit 169 a-169 n may be implemented asmultipliers. The circuit 163 may include a sampler configured to samplethe intermediate signal in response the clock signal CLK0.

In one example, the circuit 167 or the circuit 163 may be configured torun at half rate or some other rate lower than the full rate by using annT architecture (e.g., n parallel branches). The circuit 160 isillustrated implementing direct feedback. However, the circuit 160 maybe fully unrolled, or partially unrolled (some taps unrolled while theremaining taps use the summing node 167). The detector 163 and thecircuits 165 a-165 n are not generally considered part of the data ISIcanceler 161. Only the feedback multipliers 169 a-169 n and the summingnode 167 are generally considered part of the data ISI canceler. Forunrolled DFE (either fully unrolled or partially unrolled), theunrolling may be done inside the detector or before the detector,similar to the crossing ISI canceler illustrated in FIGS. 11 and 12. Anexample of a partial unrolling before the detector may be foundillustrated in FIG. 13, where the first tap is unrolled. Twocomparators, one with a threshold of H1 and another with a threshold of−H1, may be used to the accomplish the unrolling. In general, the numberof taps unrolled may be any number from 0 to N.

Referring to FIG. 9, a diagram of a circuit 170 is shown illustratinganother example of a data ISI canceler in accordance with an embodimentof the present invention. In one example, the circuit 170 may comprise ablock (or circuit) 171, a block (or circuit) 172, and a number of blocks(or circuits) 173 a-173 n. The block 171 may comprise a block (orcircuit) 174, a pair of blocks (or circuits) 175 a and 175 b, a block(or circuit) 176, and a number of blocks (or circuits) 177 b-177 n. Thecircuits 171-177 n may represent modules and/or blocks that may beimplemented as hardware, software, a combination of hardware andsoftware, or other implementations. The circuit 171 may implement a dataISI canceler. The circuit 172 may implement a detector. The circuit 173a-173 n may be implemented as storage or delay elements (e.g., shiftregisters, sample and hold elements, etc.). The circuit 174 may beimplemented as an adder. The circuits 175 a and 175 b maybe implementedas comparators. The circuit 176 may be implemented as a multiplexer. Thecircuits 177 b-177 n may be implemented as multipliers. The circuit 171may include a sampler configured to sample an output of the circuit 174in response the clock signal CLK0. FIG. 9 generally illustrates anexample where unrolling is done before the detector. The comparators 175a and 175 b, which are part of the data ISI canceler, may be usedinstead of direct feedback for the first tap. A sampler may beimplemented within the data ISI canceler, either before the summing node174 (e.g., in an analog-to-digital converter (ADC)) or after the summingnode 174. In an analog implementation of the circuit 170, the sampler,comparators and detector may be implemented together in a capture latch.In a digital signal processing (DSP) implementation, the sampler may beimplemented in the ADC, and the comparators may or may not be combinedwith the detector.

Referring to FIGS. 10 through 13, diagrams are shown illustratingdifferent examples of crossing ISI cancelers in accordance with exampleembodiments of the present invention. FIG. 10 generally illustrates acircuit 180 implementing a direct feedback version. FIG. 11 generallyillustrates a circuit 190 implementing a fully unrolled version. FIG. 12generally illustrates a circuit 200 implementing an example of a hybrid,with the first tap unrolled in the detector. In general, any number oftaps (e.g., 0 to N) may be unrolled. Unrolling is generally used whenthere is enough time for the feedback waveform to settle to an intendedmagnitude. For example, the term H0.5*DK(1) has only T/2 to settlebefore XK needs to be generated. When this is not possible, the firsttap may be unrolled.

FIG. 13 generally illustrates a circuit 210 implementing an examplewhere the unrolling is done before the detector. Comparators, which arepart of the crossing ISI canceler, may be used instead of directfeedback for the first tap. A sampler may be implemented within thecrossing ISI canceler, either before the summing node (e.g., in ananalog-to-digital converter (ADC)) or after the summing node. In ananalog implementation of the circuit 210, the sampler, comparator anddetector may be implemented together in a capture latch. In a digitalsignal processing (DSP) implementation, the sampler may be implementedin the ADC, and the comparator may or may not be combined with thedetector. Referring again to FIGS. 10 through 13, the shift registers(185 a-185 n, 194 a-194 n, 205 a-205 n, and 213 a-213 n) and detectors(183, 196 a-196 m, 202 a-202 b, and 212) are generally not part of thecrossing ISI cancelers. The shift registers (185 a-185 n, 194 a-194 n,205 a-205 n, and 213 a-213 n) are generally shared with the data ISIcanceler. The examples shown in FIGS. 10 through 13 generally illustratefull rate implementations. In general, an nT architecture may be usedfor either the summing nodes, the detectors, or both summing nodes anddetectors. The clock signal CLK90 is earlier than (leads) the clocksignal CLK0. The phase between the clock signals CLK90 and CLK0 may be90 degrees or some other values. The term H0.5 is generally defined bythe equation H0.5=P0.5−P−0.5.

Referring again to FIG. 11, N previous symbols may be used to select oneoutput from 2 ^N detector outputs. In one example, a multiplexer 198(MUX) may be configured to select between the outputs from the 2 ^Ndetectors 196 a-196 m based upon the N previous symbols DK(1), . . . ,DK(N). A threshold of each of the detectors 196 a-196 m may be one ofthe linear combinations of the terms H0.5, H1.5, . . . , HN−0.5. Forexample, one threshold may be H0.5+H1.5+. . . +HN−0.5.

Referring again to FIG. 12, the threshold for the first detector 202 amay be set as −H0.5 and the threshold for the second detector 202 b maybe set as H0.5. The detectors 196 a-196 m in FIG. 11 and the detectors202 a and 202 b in FIG. 12 are generally configured to perform both acomparison with the respective threshold, and a conversion to symbols(e.g., one bit for NRZ, 2-bits for PAM4, etc.). The inputs to themultiplexer 198 in FIG. 11 and the multiplexer 204 in FIG. 12 aregenerally already symbols.

Referring again to FIG. 13, a diagram is shown illustrating an examplewhere the detector is after the multiplexer and is separated from thecomparators. In one example, the input to the multiplexer 216 may be a7-bit signal, but the output of the detector 212 may be 2-bits for PAM4.

Referring to FIG. 14, a diagram is shown illustrating a circuit 220implementing a crossing ISI canceler in accordance with anotherembodiment of the present invention. In one example, the circuit 220 maycomprise a block (or circuit) 221, a block (or circuit) 223, and anumber of blocks (or circuits) 225 a-225 n. The block 221 may comprise ablock (or circuit) 227 and a number of blocks (or circuits) 229 a-229 n.The circuits 221-229 n may represent modules and/or blocks that may beimplemented as hardware, software, a combination of hardware andsoftware, or other implementations. The circuit 221 may implement acrossing ISI canceler. The circuit 223 may implement a detector. Thecircuit 225 a-225 n may be implemented as storage or delay elements(e.g., shift registers, sample and hold elements, etc.). The circuit 227may be implemented as an adder. The circuit 229 a-229 n may beimplemented as multipliers. The circuit 223 may include a samplerconfigured to sample the intermediate signal in response the clocksignal CLK0.

A delay line 230 may be implemented inside the crossing ISI canceler 221to allow the same sampling clock CLK0 to be used for both XK and DK. Inaddition to fewer sampling clocks, the delay line 230 generally providesa benefit in that when XK is processed, XK is already one T after DK(1),rather T/2. This gives H0.5*DK(1) enough time to settle before thecrossing sampling point. Consequently, the use of a delay line 230provides an alternative solution to the unrolling of the first tap whenthe settling time cannot be less than T/2. The delay (e.g., Tdx) of thedelay line 230 is generally set to correspond to 90 degrees of phasedifference. However, other delay values may be implemented accordinglyto meet the design criteria of a particular implementation.

Referring to FIG. 15, a flow diagram is shown illustrating an exampleprocess (or method) 300 in accordance with an embodiment of the presentinvention. The method 300 may comprise a step (or state) 302 and a step(or state) 304. In the step 302, the process 300 may generate a datasample signal by sampling a first point in a symbol interval of an inputsignal while minimizing ISI at the first sampling point. In the step304, the process 300 may generate a crossing sample signal by samplingat a second point in the symbol interval of the input signal whileminimizing ISI at the second sampling point.

Referring to FIG. 16, a flow diagram is shown illustrating anotherexample process (or method) 310 in accordance with an embodiment of thepresent invention. The method 310 may comprise a step (or state) 312, astep (or state) 314, a step (or state) 316, and a step (or state) 318.In the step 312, the process 310 may begin by generating a firstintermediate signal in response to either an input signal or a secondintermediate signal using a data ISI canceler. In the step 314, theprocess 310 may generate a second intermediate signal in response toeither the input signal or the first intermediate signal using acrossing ISI canceler. In the step 316, the process 310 may generate adata signal in response to the first intermediate signal. In the step318, the process 310 may generate a crossing sample signal in responseto the second intermediate signal.

Referring to FIG. 17, a diagram of a system 500 is shown illustrating acontext in which embodiments of the present invention may beimplemented. In one example, the system 500 may be part of acommunication system. The system 500 may comprise a transmitter 502, acommunication channel 504, and a receiver 506. The communication channel504 generally couples the transmitter 502 with the receiver 506. Thereceive 506 may comprise a block (or circuit) 510 and a block (orcircuit) 512. The circuit 510 may implement inter symbol interference(ISI) cancellation in accordance with various embodiments of the presentinvention. For example, the circuit 510 may be configured to minimizedata and crossing ISI. The circuit 512 may implement a detector inaccordance with various embodiments of the present invention. Forexample, the circuit 512 may be configured to generate data and crossingsamples. In one example, the receiver 506 may recover data transmittedfrom the transmitter 502 in response to the data and crossing samplesgenerated by the circuit 512. For example, the receiver 506 mayimplement bang-bang clock and date recovery (CDR).

The method and/or apparatus in accordance with example embodiments ofthe present invention generally operate to minimize the data ISI andcrossing ISI at the same time, so that both vertical and horizontal eyemargins may be maximized, to enable good performance and robustness. Theclosed form of the ISI at the crossing may be expressed by the followingequation:XK= . . . +(P0.5−P−0.5)*DK(1)+P1.5*DK(2)+P2.5*DK(3)+ . . .To minimize XK, the ISI from previous data samples (e.g., DK(1), DK(2),etc.) should be canceled. A dedicated crossing ISI canceler may beimplemented similarly to a decision feedback equalizer (DFE) withfeedback from the previous data samples. However, the tap weights of thededicated crossing ISI canceler may be set based on the estimates of thechannel pulse response at the 0.5UI (unit interval), 1.5UI, etc. Thededicated crossing ISI canceler may be used alone or in addition to aDFE. When the dedicated crossing ISI canceler is used in addition to theDFE, the DFE may be used to minimize the data ISI, while the dedicatedcrossing ISI canceler is used to remove the crossing ISI. When thededicated crossing ISI canceler is used in addition to the DFE, bothvertical and horizontal margins may be maximized. When the dedicatedcrossing ISI canceler is used in addition to the DFE, a separate summingcircuit may be used either in parallel or subsequent to the summingcircuit used by the DFE. The separate summing circuit may reduce crosscoupling of the DFE and the dedicated crossing ISI canceler at the dataand crossing samples.

The functions performed by the diagrams of FIGS. 15 and 16 may beimplemented using one or more of a conventional general purposeprocessor, digital computer, microprocessor, microcontroller, RISC(reduced instruction set computer) processor, CISC (complex instructionset computer) processor, SIMD (single instruction multiple data)processor, signal processor, central processing unit (CPU), arithmeticlogic unit (ALU), video digital signal processor (VDSP) and/or similarcomputational machines, programmed according to the teachings of thepresent specification, as will be apparent to those skilled in therelevant art(s). Appropriate software, firmware, coding, routines,instructions, opcodes, microcode, and/or program modules may readily beprepared by skilled programmers based on the teachings of the presentdisclosure, as will also be apparent to those skilled in the relevantart(s). The software is generally executed from a medium or severalmedia by one or more of the processors of the machine implementation.

The present invention may also be implemented by the preparation ofASICs (application specific integrated circuits), Platform ASICs, FPGAs(field programmable gate arrays), PLDs (programmable logic devices),CPLDs (complex programmable logic device), sea-of-gates, RFICs (radiofrequency integrated circuits), ASSPs (application specific standardproducts), one or more monolithic integrated circuits, one or more chipsor die arranged as flip-chip modules and/or multi-chip modules or byinterconnecting an appropriate network of conventional componentcircuits, as is described herein, modifications of which will be readilyapparent to those skilled in the art(s).

The present invention thus may also include a computer product which maybe a storage medium or media and/or a transmission medium or mediaincluding instructions which may be used to program a machine to performone or more processes or methods in accordance with the presentinvention. Execution of instructions contained in the computer productby the machine, along with operations of surrounding circuitry, maytransform input data into one or more files on the storage medium and/orone or more output signals representative of a physical object orsubstance, such as an audio and/or visual depiction. The storage mediummay include, but is not limited to, any type of disk including floppydisk, hard drive, magnetic disk, optical disk, CD-ROM, DVD andmagneto-optical disks and circuits such as ROMs (read-only memories),RAMs (random access memories), EPROMs (electronically programmableROMs), EEPROMs (electronically erasable ROMs), UVPROM (ultra-violeterasable ROMs), Flash memory, magnetic cards, optical cards, and/or anytype of media suitable for storing electronic instructions.

The elements of the invention may form part or all of one or moredevices, units, components, systems, machines and/or apparatuses. Thedevices may include, but are not limited to, servers, workstations,storage array controllers, storage systems, personal computers, laptopcomputers, notebook computers, palm computers, personal digitalassistants, portable electronic devices, battery powered devices,set-top boxes, encoders, decoders, transcoders, compressors,decompressors, pre-processors, post-processors, transmitters, receivers,transceivers, cipher circuits, cellular telephones, digital cameras,positioning and/or navigation systems, medical equipment, heads-updisplays, wireless devices, audio recording, storage and/or playbackdevices, video recording, storage and/or playback devices, gameplatforms, peripherals and/or multi-chip modules. Those skilled in therelevant art(s) would understand that the elements of the invention maybe implemented in other types of devices to meet the criteria of aparticular application.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the scope of the invention.

The invention claimed is:
 1. An apparatus comprising: an inter symbolinterference (ISI) cancellation circuit configured to minimize ISI atdata sampling and crossing sampling points in a symbol interval of aninput signal; and a detector circuit configured to generate data samplesand crossing samples at the data sampling and crossing sampling pointsin the symbol interval of the input signal, wherein each crossing sampleis generated in response to a sample of an output signal of the ISIcancellation circuit sampled at the crossing sampling point in thesymbol period of the input signal, a first threshold, a secondthreshold, and a prior data sample.
 2. The apparatus according to claim1, wherein said inter symbol interference (ISI) cancellation circuitcomprises: a data ISI cancellation circuit configured to generate afirst intermediate signal in response to either the input signal or asecond intermediate signal; and a crossing ISI cancellation circuitconfigured to generate the second intermediate signal in response toeither the input signal or the first intermediate signal.
 3. Theapparatus according to claim 2, wherein said data ISI cancellationcircuit comprises a decision feedback equalizer (DFE).
 4. The apparatusaccording to claim 2, wherein said crossing ISI cancellation circuitcomprises: an adder configured to generate an output signal in responseto a signal presented to an input of said crossing ISI cancellationcircuit and a plurality of feedback signals; at least one pair ofcomparators, said pair of comparators comprising a first comparatorhaving said first threshold and a second comparator having said secondthreshold; a sampler configured to sample the output signal in responseto a clock signal and present samples to the first and the secondcomparators; at least one multiplexer configured to select between anoutput of the first comparator and an output of the second comparator inresponse to a data sample from said input signal; and a plurality ofmultipliers, each configured to generate a respective one of saidplurality of feedback signals in response to a respective data samplefrom said input signal and a respective tap weight.
 5. The apparatusaccording to claim 2, wherein said detector circuit comprises: a firstdetector having said first threshold; a second detector having saidsecond threshold; and a multiplexer configured to select between anoutput of the first detector and an output of the second detector as acrossing sample in response to a prior data sample from said firstintermediate signal.
 6. The apparatus according to claim 5, wherein saiddetector circuit further comprises: a sampler configured to sample saidsecond intermediate signal and present the samples to said first andsaid second detectors.
 7. The apparatus according to claim 2, whereinsaid detector circuit comprises: a first detector circuit configured togenerate a data signal in response to the first intermediate signal; anda second detector circuit configured to generate a crossing samplesignal in response to the second intermediate signal.
 8. The apparatusaccording to claim 1, wherein said ISI cancellation circuit comprises:an adder configured to generate said output signal in response to saidinput signal and a plurality of feedback signals; a plurality ofmultipliers, each configured generate a respective one of said pluralityof feedback signals in response to a respective sample of said outputsignal and a tap weight.
 9. The apparatus according to claim 1, whereinsaid apparatus is part of a receiver.
 10. The apparatus according toclaim 1, further comprising a bang-bang clock and data recovery (CDR)circuit configured to adjust a phase of a sampling clock of a receiverbased upon the data and crossing samples generated by said detectorcircuit.
 11. A method of crossing ISI cancellation comprising:minimizing inter symbol interference (LSI) at data sampling and crossingsampling points in a symbol interval of an input signal; and generatingdata samples and crossing samples at the data sampling and crossingsampling points in the symbol interval of the input signal, wherein eachcrossing sample is generated in response to a sample of the input signalsampled at the crossing sampling point in the symbol period of the inputsignal following ISI minimization, a first threshold, a secondthreshold, and a prior data sample.
 12. The method according to claim11, further comprising: generating a first intermediate signal inresponse to either the input signal or a second intermediate signal; andgenerating the second intermediate signal in response to either theinput signal or the first intermediate signal.
 13. The method accordingto claim 12, further comprising: generating a data signal in response tothe first intermediate signal; and generating a crossing sample signalin response to the second intermediate signal.
 14. The method accordingto claim 11, wherein generating said crossing samples comprises:generating a plurality of detector output signals, wherein each detectoroutput signal is based upon a different threshold value; and selectingone of the plurality of detector output signals as said crossing samplesignal based upon a plurality of previously recovered symbols.
 15. Themethod according to claim 14, wherein the threshold values with whicheach of said plurality of detector output signals is generated aredetermined based upon a linear combination of terms from an estimatedchannel pulse response.
 16. The method according to claim 12, whereingenerating the second intermediate signal further comprises: generatingan output signal in response to the input signal and a plurality offeedback signals; sampling said output signal in response to a clocksignal; comparing a sample of said output signal to said firstthreshold; comparing said sample of said output signal to said secondthreshold; selecting between a result of the comparison with said firstthreshold and a result of the comparison with said second threshold inresponse to a prior data sample from said input signal; and generatingsaid plurality of feedback signals in response to previous data samplesof said input signal and a plurality of tap weights.
 17. The methodaccording to claim 12, wherein generating data samples and crossingsamples at the data sampling and crossing sampling points in the symbolinterval of the input signal comprises: sampling said secondintermediate signal; detecting a first crossing sample value based uponsaid first threshold; detecting a second crossing sample value basedupon said second threshold; and selecting between said first crossingsample value and said second crossing sample value in response to aprior data sample from said first intermediate signal.
 18. The methodaccording to claim 12, wherein generating the first intermediate signalcomprises performing decision feedback equalization on either the inputsignal or the second intermediate signal.
 19. The method according toclaim 12, wherein said second intermediate signal is generated using acrossing ISI cancellation circuit.
 20. An apparatus comprising: adetector circuit configured to generate data samples and crossingsamples at data sampling and crossing sampling points in a symbolinterval of an input signal; and a data inter symbol interference (ISI)cancellation circuit configured to minimize ISI at the data samplingpoints in the symbol interval of the input signal, wherein said data ISIcancellation circuit comprises (i) an adder configured to generate anoutput signal in response to a signal presented to an input of said dataISI cancellation circuit and a plurality of feedback signals, (ii) afirst comparator having a first threshold, (ii) a second comparatorhaving a second threshold, (iii) a sampler configured to sample theoutput signal in response to a clock signal and present samples to thefirst and the second comparators, (iv) at least one multiplexerconfigured to select between an output of the first comparator and anoutput of the second comparator in response to a data sample from saiddetector circuit, and (v) a plurality of multipliers, each configured togenerate a respective one of said plurality of feedback signals inresponse to a respective data sample from said detector circuit and arespective tap weight.